Tube plates glowing Red, amp keeps blowing fuses

[telegibstrat]

Hi everybody.
I am new to this forum and need anybody's help at this point.
A couple days ago I put some chinesee 6l6gc into my Rivera Knucklehead's. It is a 1994 model and it came with El34's.
Supply voltage is 450v on this amp. I measured current plate for the 6l6gc once installed in the amp and it came out to be 42.6 Ma on the lowest bias trim pot setting. (I thought it was hot but the amp could get by). A couple of days later the tubes started to smell funny and glow red but not that red. Yesterday the amp started blowing fuses and plates started glowing red hot all over.

Well that amp went down. I was so stupid as to use the same tubes in a Blackface tremolux that I was rebuilding and guess waht?? It happened again.

It was a painful and stupid way to find out that the tubes were damaged. My question is I now dont know whatto look for
Both amps keep blowing fuses.

If you install any tube on the Rivera now they will glow red and will eventually blow the fuse.
On the fender it only blow fuses like 10 seconds after it is turned on.

thanks very much for your help

[Don Moose]

First - Red plates == BAD - shut down immediately.

Second - either the bias didn't stay where you set it, or the 6L6s were weak, or there was an error in your measurment (did you take screen dissipation into account?).

You can move the range of bias adjustment by adding or changing a resistor in the bias supply.

Please be careful - redplating kills tubes, and can (in some cricumstances) kill OTs and PTs.

Much advice at R.G.Keen's Tube Amp Debug Page

[telegibstrat]

Thanks but where do I go from here? What damage components should i lkook for? At least in the Tremolux?

thanks very much

[Enzo]

Yes, please read through RGs debugger.

I would also just add, When I face such repairs, I remove the power tubes entirely, then I take voltage readings at the empty socket pins. Is there several hundred volts on pins 3 and 4 of every power tube socket? Is there negative bias voltage at every pin 5?

[telegibstrat]

Thanks Enzo will do that tomorrow and will let you know

thanks very much

[telegibstrat]

Hi there again

I am getting around 490V for every pin 3 and 4 of both 6l6gc on the Super Reverb. I made a mistake and said that it was a Tremolux but it was a 1967Super Reverb AB763 circuit.

There is no negative voltage pins 5 of each tube. I am getting around 3v and 2v (first tube and second tube respectively).

I will be working on this amp first and later on the other one (Rivera)
Could it be the power supply section?
I measured the bias pot= 12k fine, bias pot resistor 50k (33k out of spec),and there is also no negative voltage on the bias pot input , it should be -52 and it came out to be 0.8v

thanks for your help again

[Don Moose]

There is no negative voltage pins 5 of each tube. I am getting around 3v and 2v (first tube and second tube respectively).

Could be the bias voltage source or you have leaky PI coupling caps.

There should be a 220k (or so) resistor from each pin 5 that meet at a common point. That common point should be a negative voltage - if it is, your bias source is probably OK and you probably have leaky PI coupling caps. If not, you have a problem with your bias rectifier/filter chain.

Hope this helps!

[telegibstrat]

Voltage in this common point is .8 volts (positive). So the problem should be in the Filters of its power supply? I dont know if thie following info helps, but I sucessfully discharged them after I measured voltages today (without power tubes installed). I saw them spark.

Thanks again

[tubeswell]

Pull out all the tubes and see if you have an AC (measured VAC to ground) supply from your PT bias tap/winding. If that is okay, then your PT is still fine. The next step is check the VDC after the bias (reversed) diode. This should be a jumpy negative (unfiltered but half-wave -ve rectified) voltage without the bias supply filter cap hooked up. Once you have hooked up the bias supply filter cap, the -VDC should stablise at a fairly smooth negative voltage (which should be more than the -ve voltage you require at the output tube grids). If it doesn't, either replace the diode (making sure the anode/banded end is pointing to the PT winding), or replace the bias supply filter cap (which, if its an electrolytic cap, should have the +ve end pointing to ground, and should be rated at a greater voltage than the Bias supply provides). After that, check the resistors in the bias supply voltage divider, and the output tube grid-load resistors, to make sure they measure the Rs that they are supposed to measure.

[telegibstrat]

I got only 52.4 ACV on the supply from its PT bias tap/winding.
Is that OK?

thanks again

[tubeswell]

I got only 52.4 ACV on the supply from its PT bias tap/winding.
Is that OK?

thanks again

Yep sounds like a 'normal' 50V tap. With a 1N4007 reverse-biased (anode/banded end to PT) diode and a (say) 150V 10uF electrolytic cap (+ve end to ground) you should get about -70VDC after the bias filter cap (but before any voltage divider). If you haven't got that then either the diode or the filter cap has haddit.

[Robert M. Martinelli]

Yep sounds like a 'normal' 50V tap. With a 1N4007 reverse-biased (anode/banded end to PT) diode and a (say) 150V 10uF electrolytic cap (+ve end to ground) you should get about -70VDC after the bias filter cap (but before any voltage divider). If you haven't got that then either the diode or the filter cap has haddit.

Hi TW!
If it's a half-wave rectifier with a single reverse-biased 1N4007, then filtering it with a cap you get half of the P-P voltage, so with 50 V RMS your P-P is at about 70VAC, the diode cuts half of that ( the positive half-wave ),and you can only get about -35VDC out of the filter cap....with the bias voltage range usually going from -35 to -55 VDC I think that maybe 50VAC are not enough... to have the bias voltage at -50VDC with a single diode the RMS AC voltage out of the tranny should be about 71 VAC RMS IMHO ( RMS 71 VAC, P-P 101 VAC -----> half wave -50,5V----->1N4007 drop 0,5V------> voltage on the filter cap about -50 VDC ).

Or.....do I need to go back to school?

Hope this helps
Best regards
Bob

[Steve Conner]

Yes. Rectifying a 50V RMS bias tap will produce 70V, not 35. If we neglect the ripple (and we should, because if it wasn't negligible the amp would hum a lot) then the cap charges to the peak voltage (not P-P) which is the RMS multiplied by the square root of 2.

I know this because I spent all of Thursday morning running an Electronics 101 tutorial, and the halfwave rectifier problem was one of the things the students got stuck on.

[tubeswell]

Ooops sorry about the half-wave rectifier statement. I need to do some more swatting

(I'm pretty sure everything else I said still matters tho)

[tubeswell]

Actually, I'm still not so sure that I'm wrong about the "half-wave" bit, because the 50V tap is the equivalent of a half of a full 100V secondary (the 0V midpoint going to ground, and there is no corresponding "-50V" tap, therefore the rectifier is working on one-half of the transformer cycle). And the formula for half wave rectification is .707 x the VAC of the secondary (is it not?), so 100V x .707 = 70.7V, which is what you get.